More Information on
Urban Heat Islands

What Is an Urban Heat Island?

As urban areas develop, changes occur in their landscape. Buildings, roads, and other infrastructure replace open land and vegetation. Surfaces that were once permeable and moist become impermeable and dry.1 These changes cause urban regions to become warmer than their rural surroundings, forming an "island" of higher temperatures in the landscape.

Heat islands occur on the surface and in the atmosphere. On a hot, sunny summer day, the sun can heat dry, exposed urban surfaces, such as roofs and pavement, to temperatures 50–90°F (27–50°C) hotter than the air,2 while shaded or moist surfaces—often in more rural surroundings—remain close to air temperatures. Surface urban heat islands are typically present day and night, but tend to be strongest during the day when the sun is shining.

In contrast, atmospheric urban heat islands are often weak during the late morning and throughout the day and become more pronounced after sunset due to the slow release of heat from urban infrastructure. The annual mean air temperature of a city with 1 million people or more can be 1.8–5.4°F (1–3°C) warmer than its surroundings.3 On a clear, calm night, however, the temperature difference can be as much as 22°F (12°C).3

Surface and atmospheric temperatures vary over different land use areas.
Surface temperatures vary more than air temperatures during the day, but
they both are fairly similar at night. The dip and spike in surface
temperatures over the pond show how water maintains a fairly constant
temperature day and night, due to its high heat capacity.

* Note: The temperatures displayed above do not represent absolute
temperature values or any one particular measured heat island.
Temperatures will fluctuate based on factors such as seasons, weather
conditions, sun intensity, and ground cover.

To view images of surface and atmospheric heat islands, and to learn more about how scientists measure them, visit the Measuring Heat Islands page.

The heat island sketch pictured here shows how urban
temperatures are typically lower at the urban-rural
border than in dense downtown areas. The graphic also
show how parks, open land, and bodies of water can create
cooler areas within a city.

For additional information on urban heat islands:

read the Heat Island Basics chapter (PDF) (22 pp, 1.5MB) from EPA's Reducing Urban Heat Islands: Compendium of Strategies, which explains the different types of urban heat islands and their causes, describes the impacts of heat islands, and provides resources for more information;

watch two short video segments that EPA
developed in partnership with The Weather Channel cable
television network.

Why Do We Care About Heat Islands?

Elevated temperature from urban heat islands, particularly during the summer, can affect a community's environment and quality of life. While some heat island impacts seem positive, such as lengthening the plant-growing season, most impacts are negative and include:

Increased energy consumption: Higher temperatures in summer increase energy demand for cooling and add pressure to the electricity grid during peak periods of demand. One study estimates that the heat island effect is responsible for 5–10% of peak electricity demand for cooling buildings in cities.3

Elevated emissions of air pollutants and greenhouse gases: Increasing energy demand generally results in greater emissions of air pollutants and greenhouse gas emissions from power plants. Higher air temperatures also promote the formation of ground-level ozone.

Compromised human health and comfort: Warmer days and nights, along with higher air pollution levels, can contribute to general discomfort, respiratory difficulties, heat cramps and exhaustion, non-fatal heat stroke, and heat-related mortality.

Impaired water quality: Hot pavement and rooftop surfaces transfer their excess heat to stormwater, which then drains into storm sewers and raises water temperatures as it is released into streams, rivers, ponds, and lakes. Rapid temperature changes can be stressful to aquatic ecosystems.